Vulcanization (cross-linking) of rubber compounds in an ultrahigh frequency field, hereinafter referred to as a UHF field, has in recent years become a standard procedure in the rubber processing industry. The process has special significance for the continuous manufacture of shaped rubber articles such as extruded products. For such processes to be effective and economical it is necessary that the rubber composition be rapidly heated in the UHF field. When processing polar rubbers, e.g. nitrile rubber (NBR), the energy absorption in the high frequency alternating field is the result of the polarity of the rubber per se. However, with non-polar rubber such as ethylene-propylene-(diene) rubber [EP(D)M], natural rubber (NR), and styrene-butadiene rubber (SBR), sufficient energy absorption in the alternating electrical field is achieved only if carbon black is used as a filler. When other light-colored mineral fillers are used in the formulation of the non-polar rubber composition, the resulting formulation cannot be heated and vulcanized quickly enough due to the relatively low energy absorption of these fillers.
It is known that the energy absorption of light-colored non-polar rubber compositions can be improved by incorporating therein commonly used polar chemical compounds, for example, diethylene glycol, triethanolamine, polar plasticizers such as chlorinated hydrocarbons, as well as by the use of factices (J. Ippen, Rubber Chem. Technol., 44 (1971), No. 1, 294-306). Such additives, however, have a marked adverse affect on the vulcanization characteristics and/or the properties of the resulting vulcanizate when used in the quantities necessary to achieve sufficiently rapid heating in the UHF field.
It is further known that by mixing a polar rubber with a non-polar rubber that an appreciable increase in the energy absorption in the UHF field can be achieved. Nitrile rubber, however, is not a suitable blend component with, for instance, ethylene-propylene-diene rubbers since it adversely affects the properties of the EPDM vulcanizate.
Accordingly there is still a need for a process for quickly heating and vulcanizing non-polar rubbers, such as EPDM, in a UHF field so that the desirable properties of the rubber are maintained after vulcanization.